Circular dichroism elliptically polarized light

Because of absorption, the intensity of the plane-polarized light will also decrease exponentially with distance. In optically active media, the two components El and Er will, therefore, not only show different velocities but they will also be absorbed to a different extent. This phenomenon is termed circular dichroism (CD). The head of the resulting vector E will appear to move on an ellipse. Thus, elliptically polarized light is produced from linearly polarized light in a medium showing circular dichroism. 

Recent papers on the relationship between optical rotatory dispersion and circular dichroism of optically active systems have stressed the superiority of the latter in interpreting the data. Unfortunately, circular dichroism measurements have been either difficult or expensive to obtain. Recently available instruments are costly and the reliability of the data these instruments afford certainly has not reached its maximum. Universal polarimeters have been described, but they require either major changes in existing insttuments or special construction of new instruments. This paper describes a method of modifying the RUDOLPH Spectropolarimeter, Model 200 AS, to measure the ellipticity of elliptically polarized light and thus indirectly measure circular dichroism. This modification requires no major changes and virtually no expense. 

Optically active chromophores show different absorption for left and right circular polarized light (where the orientation of the polarized light changes periodically). These substances modify a circular polarized beam in such a way that the light is elliptically polarized after leaving the sample, an effect called circular dichroism. 

Optical activity also manifests itself in small differences in the molar extinction coefficients el and er of an enantiomer toward the right and left circularly polarized light. The small differences in e are expressed by the term molecular ellipticity [9 J = 3300(el — r). As a result of the differences in molar extinction coefficients, a circularly polarized beam in one direction is absorbed more than the other. Molecular ellipticity is dependent on temperature, solvent, and wavelength. The wavelength dependence of ellipticity is called circular dichroism (CD). CD spectroscopy is a powerful method for studying the three-dimensional structures of optically active chiral compounds, for example, for studying their absolute configurations or preferred conformations.57... 

A compound is considered optically active if it shows the phenomena of circular birefringence and circular dichroism. These are the manifestations of different refractive indices, /i, and /Ir, and different extinction coefficients, , and Cr, respectively, for left-handed and for right-handed circularly polarized light. Optical activity is therefore closely related to the existence of elliptically and circularly polarized light. 

Chiral substances also show differential absorption of circularly polarized light. This is called circular dichroism (CD) and is quantitatively expressed as the moleeular ellipticity 0, where and are the extinction coefficients of left and right circularly polarized light ... 

UV/vis light passing a solution of a chiral chromophore becomes elliptically polarized. Circular dichroism (CD) spectra correspond to the difference of the absorption coefficients of left- and right-handed polarized light. The unit of CD spectra is molar ellipticity and is given in deg cmVdmol. Single molecules with chiral centers close to the chromophore have molar ellipticities of a few hundred to thousands helical chromophores or helical assemblies of chromophores reach several millions (Hesse et al., 1984). 

Exciton-coupled circular dichroism (ECCD) spectroscopy was performed on the Aib-Stb-D peptide to fiirther substantiate the interaction between the methyl stilbene side-chains, which was observed via PL experiments. The interaction between the excited state of chromophores in a chiral environment causes split Cotton effects upon absorption of circularly polarized light by the chromophores (29, 30). It can be observed from Figure 7 that Aib-Stb-D exhibits a split CD Cotton effect presumably because of the chiral presentation of the methyl stilbene molecules on the same side of the a-helical peptide the asymmetric nature of the split observed may be due to some other electronic transitions or of additional background ellipticity, as has been observed in other systems (29, 31). The ECCD results confirm the close proximity of the methyl stilbene side chains mediated by the peptide backbone, which permits interaction between the side-chains in the excited state. 

Circular dichroism (CD) spectroscopy is based on the difference of absorption of right and left circularly polarized radiation by chiral (dissymmetric) molecules. In peptide research CD spectra can give valuable information on the conformation of peptide molecules in solution. The differences in absorption (ellipticities, 9) are pronounced in the 200 to 220 nm region. The positive or negative Cotton effects allow empirical statements on the arrangement of peptide chains, through comparisons with the CD spectra of peptides that have helical, pleated sheet or random coil conformations, established by other means, e.g. by X-ray diffraction studies. Optical rotatory dispersion (ORD) spectroscopy, which is based on the different refraction of circularly polarized light by chiral compounds, is the older... 

Optical activity of a substance leads at least to three spectroscopic phenomena which are, however, related Optical Rotatory Dispersion (ORD), i.e. the rotation of the plane of linearly polarized light by traversing the substance. Circular Dichroism, expressed as Ae = e - e (if the relative molar mass of the molecules is known, or - in case of biopolymers - the mean residual relative molar mass), and Molar Ellipticity, [e] = 3300 x Ae. A necessary condition for optical activity of a substance is the Chirality of its molecules, i.e. the property of being not superposable onto its mirror image. This allows still the presence of (proper) axes of rotation Cp, but not improper ones (Sp, including... 

This difference (dD), calculated for a molar solution and one centimeter path-length is the molar circular dichroism As, Quite often instead of AT> rather molar ellipticity, [0] is shown in graphical representations of cd spectra. This corresponds to the ratio between the short and long axes of the ellipse of the emerging (initially circular) polarized light and is directly proportional to molar circular dichroism... 

Inside an absorption region, the appearance of a circular dichroism spectrum, coupled with the conversion of linear polarized light into elliptically... 

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